The present invention relates to a method of obtaining a pure aromatic hydrocarbon from a sump product of an extractive distillation of a starting mixture containing hydrocarbons.
A method of obtaining a pure aromatic hydrocarbon from a sump product of an extractive distillation of a starting mixture containing hydrocarbons is known, in which the sump product from the extractive distillation column is fed into a separator distillation column provided with plates, a separator distillation column sump and associated auxiliary boilers. The pure aromatic hydrocarbon is distilled from the top of the separator distillation column. The solvent is drawn from the sump of the separator distillation column and, after indirect heat exchange with other process flows, is fed to the solvent input of the extractive distillation column.
Extractive distillation today is a widely used process for obtaining pure aromatic hydrocarbons from starting mixtures containing hydrocarbon compounds. For performing this kind of extractive distillation process a number of different selective solvents are currently used, including N-substituted morpholines, especially N-formyl morpholine, which have proven to be especially useful. The separating effect of the selective solvent is based on the fact that, because of its presence, the vapor pressure of the individual components of the starting mixture is changed in such a way that the vapor pressure differences between the components, which should be obtained in the extract in the sump product and the components in the top product are increased. Because of that, nonaromatic hydrocarbons leave the extractive distillation column from the top as a lower boiling fraction, while the aromatic hydrocarbons are taken from the extractive distillation column as a higher boiling fraction together with the solvent as a sump product. To separate the pure aromatic hydrocarbon from the selective solvent, the sump product of the extractive distillation column is distilled in a subsequent separator distillation column.
Continuous separation of the aromatic hydrocarbon from the sump product of the extractive distillation column requires a high heat input. The input heat is required principally for separation of the aromatic hydrocarbon from the solvent in a lower portion of the separator distillation column and for distillative separation of the aromatic hydrocarbon from the solvent residue in an upper portion of the separator distillation column. The heat required during operation of the separator distillation column accounts for a considerable portion of the operating expenses of the entire process. An improved method was required for operation of the separator distillation column, which allows processing of the sump product from the extractive distillation column with less heat and thus lower energy costs. Thus an operation of the separator distillation column is desired, which is characterized by an efficient use of heat and a minimum of apparatus expense.
To perform an extractive distillation process for obtaining a pure aromatic hydrocarbon, it is known to use the heat content of the hot solvent flowing from the sump of the separator distillation column. The solvent is cooled prior to its return to the extractive distillation column in an indirect heat exchanger. Thus for example in a report by the applicant (Koppers Bericht (Report) 333 b v. IX. 69) a method for obtaining a pure o-Xylol from a reformate by extractive distillation is described, in which the solvent drawn from the separator distillation column is fed into an indirect heat exchanger for heating of the sump product of the separator distillation column and the extractive distillation column. However the solvent is not cooled enough in the indirect heat exchange with the sump product so that it can be returned directly from the indirect heat exchanger to the extractive distillation column. An additional cooling of the solvent must be performed in an air cooler, so that its heat content is further reduced. An indirect heat exchange between the sump product drawn from the extractive distillation column and other process streams is not provided in this method.